1. Field of the Invention
The invention concerns a catalyst for decreasing the content of nitrogen oxides in flue gases.
2. Background Information
Nitrogen oxides (NO.sub.x) are generated both from the nitrogenous constituents of the fuel and from the nitrogen in the air when fossil fuels are burned. The oxides enter the atmosphere and become extremely detrimental to the environment.
It is known that nitrogen oxides can be converted into N.sub.2 and H.sub.2 O by NH.sub.3 and that the reaction is fairly selective over a wide range of temperatures, meaning that, since it proceeds in the presence of a high excess of oxygen (as is usual in flue gases) without excessive loss of ammonia as the result of oxidation, only relatively small amounts of reductants are necessary. Various catalysts for reducing NO.sub.x with ammonia are also known.
German AS No. 2 410 175, for example, discloses catalysts of this type that consist of oxides of vanadium, molybdenum, and/or tungsten. The stoichiometry is V.sub.12-x-y Mo.sub.x W.sub.y, wherein 0.ltoreq.x.ltoreq.8.0.ltoreq.y.ltoreq.5 and 0.3.ltoreq.(x+y).ltoreq.8.
Furthermore, German Pat. No. 2 458 888 discloses a method of reductively decomposing nitrogen oxides in flue gases. A mixture of gases containing nitrogen oxides, molecular oxygen, and ammonia is contacted with a catalyst composition that contains (A) titanium in the form of oxides in an intimate mixture with (B) iron or vanadium in the form of oxides as its essential constituents.
The drawback to these catalysts is that the catalytically active constituents that they contain in the form of relatively expensive transition metals are exploited only to a low degree because they are not optimally distributed. Although the active constituents are extended by solid inert carriers, which does make them more economical, the dilution with inert material entails the risk of extensively decreasing their catalytic activity. Another drawback to these catalysts is that they also catalyze the SO.sub.2 that is often contained in the flue gas into SO.sub.3, which can lead for example to deposits of salts in the downstream equipment of the system.
German OS No. 3 438 367 also discloses a catalyst for decreasing the content of nitrogen oxide in flue gases by selective reduction. The catalyst consists of (A) 80 to 95% by weight of a catalytic oxide containing a sulfur oxide and is obtainable among other methods by heat treating an aqueous oxide compound of titanium or silicon, (B) 0 to 5% by weight of a catalytic oxide that contains vanadium oxide, and (C) 1 to 15% by weight of a catalytic oxide like tungsten oxide.
Considered essential to this catalyst is the formation of a solid acid composed of SiO.sub.2 and TiO.sub.2, its acidity modified by treatment with sulfuric acid or ammonium sulfate. The distribution of the solid acid is considered as representing the standard for controlling the adsorption of NH.sub.3 at the surface of the catalyst and hence for improving its catalytic activity.
The SiO.sub.2 is employed in the form of a silica sol. It is known that silica gels distinguished both by high BET surfaces and by high porosity can be obtained from SiO.sub.2 sols, although the percentage of macropores is low, which has a deleterious effect on material transport and hence on catalytic activity.
German OS No. 2 748 471, finally, discloses a catalyst composition to be employed in the vapor-phase reduction of nitrogen oxides with ammonia, especially to reduce the content of nitrogen oxides in flue gases. This catalyst consists essentially of an oxide or sulfate of one of the metals copper, vanadium, chromium, molybdenum, tungsten, manganese, iron, or cerium on a shaped carrier that contains titanium oxide and a minor portion of a clay mineral with a mean particle size of 0.1 to 100 .mu.m. Clay minerals like montmorillonite, kaolin, halloysite, pyrophillite, and sericite can be employed. These are aluminosilicates with a layered structure, some of them three-layer silicates. Up to 15% by weight of these clay minerals is claimed to increase only the stability of the catalyst. The additives have no significant effect on catalytic activity at these levels and larger amounts even have a negative effect in this respect. Due to their chemical composition, they also decrease the catalyst's resistance to flue gases that contain SO.sub.x.